This relates generally to imaging systems, and more particularly to imaging systems with analog-to-digital converters.
Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Imagers (i.e., image sensors) may be formed from a two-dimensional array of image sensing pixels. Each pixel receives incident photons (light) and converts the photons into electrical signals. Image sensors are sometimes designed to provide images to electronic devices using a Joint Photographic Experts Group (JPEG) format.
Analog-to-digital converters (ADCs) are used to convert analog signals produced by the pixels into digital signals for processing. Conventional analog-to-digital converters use a comparator to determine when a ramp signal reaches a sampled pixel reset signal. A counter is used to count how many clock cycles are between the initial application of the ramp voltage and the time at which the ramp signal matches the sampled pixel reset signal. The counter value is used as the digital output of the pixel.
Temporal noise can cause inaccuracies in pixel output values. Operation of analog-to-digital converters is a significant contributor of temporal noise and can lead to reduced imaging accuracy. Some analog-to-digital converters reduce temporal noise by collecting multiple digital samples and averaging the results. To collect multiple samples, the ramp signal is applied with a positive slope for a first sample and with a negative slope for a second sample. However, the rate of voltage change per unit time is constant and predetermined by the design of ramp generation circuitry that produces the ramp signal. Producing multiple digital samples using conventional ADC circuitry requires either twice the amount of ADC operational time at a given slope or ramp generation circuitry capable of generating a ramp signal with twice the given slope. Increases in the ADC operational time incurs processing delays and reduced performance. Ramp generation circuitry that produces twice the given slope increases power consumption and, moreover, is not always attainable.